Video pedestal network

ABSTRACT

An architecture for distributing digital information to subscriber units wherein selection from among multiple digital services is accomplished by transmitting a tuning command from a subscriber unit to an intermediate interface. The intermediate interface selects the desired service from a broadband network and transmits it to the subscriber unit over a bandwidth-constrained access line. The bandwidth-constrained access line may be implemented with existing infrastructure, yet the subscriber unit may access a wide variety of digital information available on the broadband network. Universal broadband access is thus provided at low cost. Output bandwidth of broadcast equipment may also be optimized.

STATEMENT OF RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 08/451,264, filedMay 26, 1995 Now U.S. Pat. No. 5,793,410.

BACKGROUND OF THE INVENTION

The present invention relates generally to delivery of digitalinformation to subscriber premises and more particularly to providingthis access without substantial new wiring expense.

The delivery of digital video services to the home represents oneimportant aspect of the much publicized "information highway." Theseservices include HDTV, video-on-demand (VOD), near-video-on-demand(NVOD) providing staggered program starting times, interactive videoservices (IVS), and other digital variants of conventional broadcastservices.

One of the main obstacles to the introduction of these services isdevelopment of the necessary infrastructure for delivering digital videoinformation to the subscribers' premises. Video services, even with theuse of modern compression standards such as MPEG-1 and MPEG-2,intrinsically require large bandwidths. Somehow an infrastructure mustbe constructed to distribute the necessary signals to individualsubscriber premises. At a minimum, each subscriber should be able toselect from among numerous digital video programs as can be done nowwith analog broadcast television.

Various solutions have been proposed. In one scheme, the digital dataare distributed via an asynchronous transfer mode (ATM) network to eachsubscriber's premises. The physical medium of the ATM network may beimplemented in more than one way. One way is to lay optical fiber toeach home. Alternatively, fiber may be laid up to the curb, from whichpoint a coaxial cable can relay the ATM cells. The advantages of thistechnique are the low latency and flexibility of the ATM technology anda large bidirectional bandwidth sufficient to distribute numerousinteractive digital video programs. Nonetheless, this approach ispractically infeasible today since the cost of laying fiber or coaxialcable to each home is prohibitive. Additionally, the time required todeploy such an infrastructure over a large geographic area makes thescheme even more unattractive.

An alternative scheme is the so-called hybrid fiber coax (HFC) scheme.The HFC scheme provides a two-level network. At the higher level,optical fibers are used to distribute digital information to a pluralityof Cable Headends or Host Digital Terminals (HDT). Each Headend or HDTin turn distributes information to multiple hybrid fiber coaxial cables,each of which serves several hundred subscriber units in a bus/looparchitecture. The return channel over the coaxial cable is also sharedby multiple subscriber units by employing Time Division Multiplexing(TDM). Again, for those network providers that do not already have suchan infrastructure installed, costs are prohibitive because coaxial cablemust be brought to each home. Furthermore, the use of TDM coupled withhighly limited bandwidth gives rise to a large latency in the returnchannel. Network security is another drawback of the HFC architecture asseveral users share a single coaxial cable, a particular concern forinteractive services that may require transmission of a subscriber'sprivate information.

Other schemes take advantage of the existing telephone network by usingADSL technology to transfer high data rate information, such as video,over existing telephone company twisted pair lines to subscriberpremises. Optical fiber may be used to transfer digital information tothe telephone company central office or to a curbside interface wherethe twisted pair lines begin. The latter architecture is commonlyreferred to as Fiber-To-The-Curb (FTTC). Alternatively, fiber may bedeployed till the basement of a large building, from which pointexisting twisted pair lines can establish connection with eachsubscriber. Such an architecture is commonly referred to asFiber-To-The-Building (FTTB). The disadvantage of this approach is thatADSL provides insufficient bandwidth. Most current ADSL trials carryonly 1.5 or 2 Mb/s over twisted pair. Laboratory demonstrations haveshown that in the next few years cost effective solutions that provideup to 25 Mb/s may be possible, but even this would be insufficient toprovide a broadcast or NVOD service with an acceptable number of serviceselections. Approaches which bring fiber to the curb carry the addedcost of laying the fiber.

Prior art digital data delivery schemes that use relatively narrowbandwidth connections to the subscriber premises require point-to-pointsessions between the ultimate server and subscriber unit. This isbecause the narrow bandwidth link that is closest to the subscriberpermits only a point-to-point connection if the desired service qualityis expected to be reasonable. These point-to-point sessions wastebandwidth since the server must separately transmit to multiplesubscriber units requesting the same program. If the user wishes toswitch channels, there is significant extra latency resulting from theneed to end the previous point-to-point session before beginning a newone. Furthermore, the network and server hardware needed to accommodatepoint-to-point sessions is particularly complex and expensive.

SUMMARY OF THE INVENTION

The invention provides a network architecture for distributing digitaldata to subscriber units wherein selection from among multiple digitalservices is accomplished by transmitting a tuning command from asubscriber unit to one or more intermediate interfaces in a series oflinks interconnecting the subscriber unit and a server. An example ofsuch digital data is digital video and the services could be multipledigital video programs. The network architecture of the presentinvention is capable of providing public broadband access without theuse of very high bandwidth access lines to subscriber premises.

Using the information received from the subscriber unit, theintermediate interface selects the desired digital service from amultitude of services available in a broadband link coupled to theinterface's input and transmits it to the subscriber unit over abandwidth-constrained link. The bandwidth-constrained link may beimplemented with existing infrastructure, yet the subscriber unit mayreadily access a wide variety of digital services available on thebroadband network. The present invention thus combines universalbroadband access with low cost.

In accordance with the invention, a service provider may offer broadcastservices over the network in addition to point-to-point interactiveservices. The broadcast services may be offered without requiringmultiple point-to-point sessions from the server to each requestingsubscriber unit. Instead, a single copy of a digital stream provided viathe broadband link is sufficient to service multiple subscriber unitswhich request it via their intermediate interfaces, thus conservingbandwidth. By contrast, in the prior art point-to-point schemes, thebroadband network would be forced to carry a separate point-to-pointconnection for each requesting subscriber unit.

Note that the present invention provides the advantages of forgoingpoint-to-point connections for broadcast purposes without requiring theconstruction of the very high bandwidth links necessary to continuouslytransmit each available program to each subscriber unit. Thus digitalbroadcast and NVOD services may be provided effectively and at low cost.

In accordance with one aspect of the present invention, a conventionaltelephone network is enhanced to provide universal high bandwidthdigital service. Typically, telephone service is provided to subscriberpremises via individual access lines extending from the subscriberpremises to a neighborhood hub or pedestal. The access lines are privateand secure twisted pair lines. The pedestal is fed by a high data-ratetrunk line, typically implemented as a fiber optic connection.

High bandwidth digital service, including video programming, istransmitted over a twisted pair connection by implementing AsymmetricDigital Subscriber Link (ADSL) modulation and demodulation over theconnection which allows for transmission of high speed digital data in amanner that is transparent to existing telephonic traffic, as is wellknown to those of skill in the art. The network equipment for ADSLmodulation and demodulation typically resides in the pedestal.

In accordance with this aspect of the present invention, the pedestal isfurther specially adapted to receive tuning information from asubscriber unit to which it is coupled by an access line. The pedestaluses the tuning information to select the specified digital data fromthe multitude of service data received over the broadband network at itsinputs to be relayed to the subscriber unit.

In accordance with the present invention, this architecture could befurther extended by including one or more additional interfaces. Forexample, in addition to the pedestal, an interface could be providedwithin the telephone company central office. The overall videodistribution scheme would then incorporate three interconnected layers.The lowest layer would constitute the individual narrow bandwidth accesslines between the pedestals and subscriber premises, the middle layerwould constitute an intermediate bandwidth link between the centraloffice and the pedestal, and the top layer would be a broadband networkfeeding the central office or some other broadband source, e.g., asatellite feed. With this scheme, the tuning function could be sharedbetween the pedestal and the interface within the central office. Thepedestal would respond to a tuning command from a subscriber unit byattempting to extract the desired program from the intermediatebandwidth network. If the desired program is not already available viathe intermediate bandwidth link, the pedestal signals the central officeinterface to retrieve the program from the broadband network. Of course,this multilayer architecture could be extended indefinitely over anynumber of interfaces and layers.

The capacity of the intermediate bandwidth link would limit the numberof different choices that could be selected simultaneously bysubscribers serviced by the same pedestal. For example, the intermediatebandwidth link could carry a fixed number of NVOD channels with eachsubscriber able to choose one for current viewing. However, not all ofthe NVOD channels available via the intermediate bandwidth link arelikely to be viewed simultaneously. In accordance with the invention,additional digital services may opportunistically exploit intermediatelink capacity left unused by current subscriber activity. Theseadditional services could include other broadcast and NVOD channels, VODservice, or interactive services.

In accordance with the present invention, this opportunistic bandwidthscheme could also be adapted to operate in the context of a loop or busarchitected access line network, such as a hybrid fiber/coaxial (HFC)network, providing digital broadcast, NVOD or other interactiveservices. In a HFC network a Host Digital Terminal (HDT) or CableHeadend couples a broadband optical network to one or more HFC cableseach of which serves multiple subscriber units in a loop/busarchitecture. The HDT or Cable Headend can be modified in accordancewith the invention to incorporate an interface that passes only thatdigital service that has been requested by a subscriber unit as opposedto the prior art method of carrying all the streams belonging to abroadcast or NVOD offering. If bandwidth remains on the cable,additional services can opportunistically exploit it.

Numerous other combinations of layers and interfaces are possible withinthe scope of the present invention. For example, the broadband networkmay be implemented as one or more satellite or MMDS (MultichannelMicrowave Distribution System) feeds. A video distribution system inaccordance with the present invention may also be easily extended toprovide interactive services. Alternatively, information besides videoinformation could be distributed. For example, interactive internetservices may be provided over the same network that incorporates thepresent invention.

In accordance with another aspect of the invention, statisticalmultiplexing is extended to optimize usage of the output bandwidth ofbroadcast equipment, such as NVOD servers and MPEG multiplexers. Thisaspect of the invention is independent of network architecture.

The invention will be better understood by reference to the followingdetailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a typical pedestal-based telephone network.

FIG. 2 depicts a top-level diagram of a video pedestal network inaccordance with one embodiment of the present invention.

FIG. 3 depicts how a desired program may be filtered from amulti-program stream in accordance with one embodiment of the presentinvention.

FIG. 4 depicts how two desired programs may be filtered from amulti-program stream in accordance with the invention.

FIG. 5 depicts data flow to the subscriber within a video pedestal inaccordance with one embodiment of the present invention.

FIG. 6 depicts data flow from the subscriber within a video pedestal inaccordance with one embodiment of the present invention.

FIG. 7 depicts a multiple-feed video pedestal architecture based on across-bar switch in accordance with one embodiment of the presentinvention.

FIG. 8 depicts a two layer video pedestal network architecture inaccordance with one embodiment of the present invention.

FIG. 9 depicts statistical multiplexing within a four layer videopedestal network in accordance with one embodiment of the presentinvention.

FIG. 10 depicts a video pedestal network wherein a video pedestal isadapted to incorporate input from a local server in accordance with oneembodiment of the present invention.

FIG. 11 depicts a modified NVOD server in accordance with one embodimentof the present invention.

FIG. 12A depicts a modified NVOD server connected to the video pedestalnetwork architecture of FIG. 8 in accordance with the invention.

FIG. 12B depicts a modified NVOD server directly connected to subscriberunits in accordance with the invention.

FIG. 13A depicts the operation of a prior art NVOD server.

FIG. 13B depicts the operation of a modified NVOD server in accordancewith one embodiment of the present invention.

FIG. 14 depicts an MPEG multiplexer modified in accordance with oneembodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention provides a system for distributing digitalinformation from a broadband digital information source to one or moresubscriber units. An exemplary digital information distribution systemin accordance with the present invention includes a plurality of digitalinformation servers coupled to a plurality of Subscriber Terminal Units(STUs) or subscriber units via a plurality of links.

A link provides a mechanism for transferring data from one entity,called the source, to another entity, called the receiver. A link iscapable of transferring data in either direction or both directionssimultaneously. An entity that is a receiver for data travelling alongone direction in a link can be the source for data travelling along theopposite direction in the same link. Likewise, an entity that is areceiver for one link can be a source for another link. For example, ifdata passes down one link into a receiver and continues into anotherlink, the same entity that was the receiver for the first link becomesthe source for the second link.

Multiple receivers may be coupled to a common source via the same link.Similarly, multiple sources may be coupled to a common receiver via thesame link. An example of these two implementations is a bus or looparchitecture. In another architecture, commonly referred to as the stararchitecture, one link couples only one receiver to one source (ofcourse each entity can be either the receiver or the source or even bothat one time since data may travel bi-directionally).

Digital data travels from the information servers, which in this contextconstitute the broadband digital information source, to the STUs via twoor more links and possibly via one or more receivers/sources between thelinks. All receivers/sources through which data travels before reachingthe STU may be referred to as intermediate interfaces or simplyinterfaces.

The digital data provided by the information servers may be in the formof text, graphics, audio or sound, video or still image, or binarycomputer data. It should be noted that the STU can be viewed as areceiver or source as well as these terms have been used in thediscussion above. The function of the STU or the subscriber unit is toreceive the desired data from the network and convert it to a formuseable by the subscriber, e.g., a video stream may be displayed on aconventional television screen, or a text stream may be printed onpaper. The information servers may be general purpose media servers,video servers, real-time audio and video encoders, or any other devicefrom which digital data may be extracted.

Links may be implemented in a variety of physical media withoutrestriction. For example, a link may be a satellite channel, an opticalfiber channel, a coaxial cable, a telephone twisted pair, or a microwavechannel.

In accordance with the invention, the subscriber unit may select aparticular stream of digital information to be received from manystreams of digital information available from a source even if theavailable streams would not together fit within the bandwidth of thelink connected to the subscriber unit. This is done by shifting theselection or tuning function away from the subscriber unit to one ormore interfaces between the links or between a link and the source. Thetuning function is directly controlled by the subscriber unit.

The subscriber unit sends a message to a particular interface selectinga desired stream of digital information. The interface responds to themessage by selecting the desired stream from a multitude of streamsreceived at that interface from the source and relaying that streamtoward the subscriber unit via one or more links. In a system withmultiple links and interfaces between them, the interface to which themessage is directed may not itself currently have the desired digitalinformation stream available to it. Such an interface would respond byrequesting the desired stream from another interface closer to thebroadband source. The tuning function could be distributed over manyinterfaces in this way.

The digital information distribution system of the present inventionwill be made clearer with reference to a particular example, themodification of a conventional public telephone network to distributedigital video services such as digital broadcasting of standarddefinition and High Definition TeleVision (HDTV) programs,video-on-demand (VOD), near-video-on-demand (NVOD) providing staggeredprogram starting times, interactive video services (IVS), and internetservices.

FIG. 1 depicts a typical pedestal-based telephone network 100. Telephonenetwork 100 includes a series of access lines 102 connected toindividual telephone terminals, a hub or pedestal 104, and a fiber trunk106 to connect the pedestal to the central office. It should be notedthat the pedestal may in fact be within the Central Office itself, i.e.,it is not a remote unit. In such a situation, fiber trunk 106 is thecore telephone network. If the pedestals are remote units, theconnection between the Central Office and the pedestals, fiber trunk106, could either have a star topology, or it could have a bus/looptopology. Access lines 102 provide the individual customer terminalswith private and secure connections to the network. Fiber trunk 106typically carries data at OC1/DS3 (approximately 45 Mb/s) or OC3(approximately 155 Mb/s) rates, typically over an optical link.

Access lines 102 are typically copper twisted pair lines having amaximum length less than a mile. The function of pedestal 104 in thedownstream direction is to demultiplex telephone data received fromfiber trunk 106 and route it to appropriate terminals in customerpremises. For upstream operation, pedestal 104 aggregates incoming datareceived via access lines 102, multiplexes it and sends it over trunk106.

In accordance with one embodiment of the present invention, pedestal 104is modified so that digital video and other digital services areprovided without laying of new access lines. A telephone networkso-modified is herein referred to as a "Video Pedestal Network" or"VPN". A pedestal so-modified is referred to as a "Video Pedestal" or"VP".

In an exemplary VPN, MPEG-2 data is delivered to the pedestal at a veryhigh rate (e.g., OC3 or higher). Data is carried to subscriber premisesvia the copper line using Asymmetric Digital Subscriber Link (ADSL)technology as known to those of skill in the art. However, since thebandwidth of the copper line is much less than that of the fiber, notall the incoming data can be transmitted over the copper line. Thisbandwidth mismatch is resolved by requiring the pedestal to filter outall MPEG-2 packets that are irrelevant to the program selected by asubscriber unit which is itself connected to the copper access line. Inother words, the pedestal sends out over each copper line only thoseMPEG-2 packets that are necessary to enable reception of the selectedprogram channel. The subscriber unit receives only a single programMPEG-2 transport stream (or alteratively, a very small set of MPEG-2transport streams) at its input.

From the above it is clear that the pedestal must somehow know whichpackets in the incoming stream are to go down through each access line.In accordance with the invention, each subscriber unit tells the VP asto which MPEG-2 packets it wishes to receive. This is accomplishedthrough a special signaling protocol between the subscriber unit and theVP. The VP generally does not need to know anything about the content ofeither the incoming or outgoing MPEG-2 stream. It receives informationfrom each subscriber unit as to which packets that subscriber unitwishes to receive and merely filters those packets down through theaccess line. As a result, the VP architecture is kept very simple. Inessence, a part of the channel tuning functionality has been shiftedfrom the subscriber unit, where it would reside for a conventional cabletelevision system or prior art digital VOD and NVOD systems, to thenetwork.

The shifting of tuning intelligence into the VP gives the VPNsignificant advantages over the so-called FTTCO (Fiber-to-the-CentralOffice), FTTB and other similar schemes of the prior art, wherein thesubscriber unit selects from among digital video data transmitted overthe twisted pair line via ADSL. In FTTCO, FTTB and other similarschemes, the subscriber unit only "sees" a small amount of MPEG-2 data,equal to whatever can be transmitted over the copper line from anoptical network unit (which couples the fiber to the low bandwidthaccess line). Since FTTCO, FTTB and other similar schemes in theircurrent implementation do not perform any tuning in the network, theircapabilities are limited to point-to-point services. In contrast, theVPN presents the subscriber unit with a virtual connection to the corenetwork (fiber trunk), thus enabling it to receive substantially moreservices, such as broadcast and NVOD, than would be available withFTTCO.

It is stressed that the broadcast and NVOD services are in addition towhatever services existing FTTC, FTTB and other existing similar schemesprovide. This is because the VPN can be architected to have the sameswitching capabilities as these prior art networks for providingpoint-to-point connections.

VPN has several advantages over the HFC network architecture as well.While it is true that HFC supports broadcast and NVOD services readily,it does not effectively support point-to-point interactive services.This is due to extremely low bandwidths available for the returnchannel. The VPN does not suffer from such a disadvantage since it isbased on a star topology. Security is also superior in the VPN. Onceagain, since the VPN uses a star topology for the access part of thenetwork, each customer has a private connection to the network. This isin contrast to the HFC architecture, where each coax is shared byseveral hundred users in a loop architecture. As a consequence there ismuch more security and privacy in the VPN.

Note that in the VPN architecture the ADSL latency also adds to thechannel acquisition time since part of the tuning is performed in theVP. However, the extra few tens of milliseconds does not represent asignificant impact on the typical 500 milliseconds or higher number thatsubscriber units typically require for locking onto a new video channel.

Because VPN as described with reference to FIG. 1 does not requirelaying of new cables or wires to every individual subscriber's premises,it is possible to launch digital broadcast and NVOD services on a largescale relatively quickly--much more quickly than for distributionsystems such as FTTCO, FTTC and FTTB. The important modifications to thetelephone network to implement the VPN are in the pedestals.

A very significant advantage of the VPN, however, is that it seamlesslyevolves to a completely interactive network once service offeringsmature. By ensuring that the signaling protocol between the subscriberunit and the VPN is very general it is possible to support all theinteractive services right from the beginning. This service evolutionfeature, combined with the relatively insignificant cost of networkconstruction, makes the VPN architecture of the present inventionextremely attractive.

The architecture of the subscriber unit that connects to the VPN is nodifferent from that of subscriber units of the prior art, except, ofcourse, for the support required for the tuning signaling protocolbetween the subscriber unit and the network. As in the case of any othernetwork, the subscriber unit connected to the VPN has a networkinterface module appropriate to the characteristics of the access lineconnecting the subscriber unit to the network, e.g., an ADSL networkinterface module will be required for a twisted pair copper access line.

FIG. 2 depicts a top-level diagram of a VPN 200 of one embodiment of thepresent invention. VPN 200 includes a representative subscriber unit202, an ADSL access network 204 having a star topology, a VP 206, a highbandwidth core network 208, a series of servers and/or encoders 209, anda network manager 210.

VP 206 interfaces core network 208 with access network 204. Core network208 is a high bandwidth digital network, or series of networks, thatinterfaces with the content servers 209. Core network 208 could be basedon fiber, satellite, or microwave, or any combination of thesetechnologies. Access network 204 is the final segment of the deliverysystem. To save cost access network 204 is preferably implemented onexisting medium such as telephone wires (copper twisted pair). Ofcourse, it is not a requirement for the present invention that theaccess lines be copper twisted pair. For example, if no existing accesslines are available as would be the case if the network provider is anentity other than a telephone company, any other physical medium that isappropriate to the needs and constraints of the network provider may beused, e.g., coaxial cable. Subscriber unit 202 interfaces with accessnetwork 204 through a network interface module (not shown).

As described above VP 206 interfaces access network 204 with corenetwork 208. In essence, VP 206 shields subscriber unit 202 from thecore network architecture, thus making it possible to use the samesubscriber unit with different core delivery systems (satellite, fiber,microwave). Of course, the part of VP 206 that interfaces with corenetwork 208 will be dependent on the characteristics of the latter. TheVP also has an interface with network manager 210 for purposes ofcontrol and monitoring.

The input to VP 206 is preferably a high bandwidth MPEG-2 streamappropriately adapted to the physical medium of core network 208. Forexample, if core network 208 is built using optical fiber, MPEG-2 datacould be carried over ATM cells using AAL5 as known to those of skill inthe art. Alternatively, if core network 208 is a satellite channel,MPEG-2 data would typically be directly modulated using Quadrature PhaseShift Keying (QPSK), again as known to those of skill in the art. Anyphysical layer could be used to implement core network 208 in accordancewith the present invention.

VP 206 has multiple physical outputs, each of which is a twisted pairtelephone line. First consider the case where only one program isrequired to be sent down over each twisted pair connection. Current ADSLtechnology limits the bandwidth of each telephone line to approximately6-8 Mb/s, which is much narrower than the bandwidth of the core network.

The principal function of VP 206 is to receive from each subscriber unitinformation as to which one program that subscriber unit wishes todecode, filter from the input stream the appropriate MPEG-2 packetscorresponding to that program, and transmit them to the subscriber unit.The process of filtering MPEG-2 packets and transmitting them over theaccess network is performed without violating any MPEG-2 requirements,and without introducing significant jitter in the MPEG-2 packets.

FIG. 3 depicts how a desired program may be filtered from amulti-program stream in accordance with the invention. The top portionof FIG. 3 depicts a high bandwidth MPEG-2 stream 302 coming into VP 206.This bitstream contains packets belonging to many different programsincluding a single program whose packets are denoted by slant-hatchedlines. The bottom portion shows a stream 304 of filtered packetsbelonging to this single program leaving VP 206. The output rate is muchlower than the input rate which results in "stretching" of the packetson the time line (since the packets in the input and output streams areall of the same size, i.e., 188 bytes as specified by MPEG-2). Note thatthe order of packets in the filtered program is not altered.

In reality, packets of disparate origin must be carried over the accessline at the same time. For example, in one embodiment that is compliantwith the European Digital Video Broadcasting (DVB) standard, bothprogram data and Service Information (SI) (which carries the electronicprogram guide) data will typically be simultaneously delivered to thesubscriber unit as known to those of skill in the art. Furthermore, withfuture advances in ADSL technology, it will become feasible to increasethe capacity of the twisted pair medium to the point that it will becomepossible to support multiple video programs over a single telephoneline. For example, with 25 Mb/s capacity it should be possible to carry5 or 6 services. This can be used to serve multiple subscriber unitswithin one home simultaneously over a single twisted pair access line.Alternatively, if the access line is implemented using a coaxial cablewhose bandwidth is higher than that of twisted pair, it may be requiredto carry more than one service over this line in order to supportmultiple subscriber units in the home.

FIG. 4 depicts filtering of MPEG-2 packets belonging to two differentprograms. The top portion of FIG. 4 depicts a high data rate MPEG-2packet stream 402 entering VP 206. The bottom portion of FIG. 4 depictsa stream 404 carrying packets belonging to two different programs. InFIG. 4, one program (denoted by shaded packets) has a rate equal to oneeighth of the total incoming rate, while the other program (denoted byslant-hatched packets) has a rate equal to one fourth of the total rate.These ratios are of course only examples. The outgoing dual-program rateequals the sum of the rates of the two filtered programs, i.e., it is3/8 times the incoming rate. The packets in the outgoing stream are inthe same order as they were in the incoming stream. By ensuring that theoutgoing rate is exactly equal to the sum of all the filtered programrates, it is possible to ensure that the received packet time stampvalues in the subscriber unit are accurate.

FIG. 5 depicts data flow to the subscriber within VP 206 in accordancewith one embodiment of the present invention. VP 206 includes a networkadapter 502, and then for each access line, an ADSL modulator 504, and aline buffer 506. There is also an MPEG-2 Packet ID (PID) Lookup Table(PLUT) 508 and filter 510 for each outgoing access line. The concept ofPID is well known to those of skill in the art. PLUT 508 contains a listof all PIDs that are to be transmitted over the access line and isconstructed based on information received from the subscriber unit.Using PLUT 508, PID Filter 510 selects the appropriate PIDs from themultitude of PIDs contained in the entering stream and sends them toline buffer 506, which is a simple FIFO. Line buffer 506 is allowed tofill up until it is half-full before packets are removed. The rate atwhich buffer 506 is emptied equals the sum of the rates of all theprograms being transmitted over the access line. For example, if 2programs are filtered, with rates of 3 and 6 Mb/s respectively, linebuffer 506 is emptied at a rate of 9 Mb/s. The access line data rate ispreferably adjustable, in real time, up to its maximum value. If theaccess line data rate is fixed and greater than the rate of the outgoingMPEG-2 stream then it may be necessary to appropriately "pad" theoutgoing stream with "null" or "dummy" packets to fill up the totalavailable bandwidth.

A side benefit is that VP 206 can also be used as a "de-jittering"device, i.e., it can be used to eliminate any jitter introduced by corenetwork 204. This eliminates the need for an extra buffer within thesubscriber unit and also improves the timing recovery performance of theMPEG-2 decoder in the subscriber unit. The de-jittering is done in linebuffer 506. Its size therefore depends on the extent of jitter in theincoming stream. If there is no jitter in the incoming bitstream, thenthis buffer need only be a few packets deep. However, if core network204 does introduce jitter (e.g., if it is a switched ATM network), thenthe size of line buffer 506 can be set equal to twice the maximum jittertimes the maximum rate on the access line. For example, assume that thecore network can introduce a maximum delay of +/-2 milliseconds. If themaximum data rate on the ADSL access line is 20 Mb/s, line buffer 506can be specified to have a size of 2*2(milliseconds)*20(Mb/s)=80,000bits.

This scheme assumes that VP 206 is aware of the data rate of theoutgoing streams on each access line 204. This is ensured by having thesubscriber units convey the stream rate to the VP, in addition toconveying the MPEG-2 PID information. The subscriber units themselvesknow the rates from the Program Specific Information (PSI) stream thatis part of the MPEG-2 data stream, as will be easily understood by thoseof skill in the art. The "rate descriptor" in the PSI stream isspecified by MPEG-2 to be an optional field. However, it is clear fromthe above statement that in this embodiment, the rate descriptor ismandatory in the PSI. In the case of MPEG-2 services that are ofvariable rate, the maximum possible rate of the service is carried inthe rate descriptor field of the PSI. Such services are relayed by theVP to the subscriber unit at their maximum rate values.

To cover the most general case, the subscriber unit signals not only thePID and rate information to the VP, but also all other information thatuniquely identifies the required stream in the core network. Forexample, in one embodiment that is compliant with the European Standardfor Digital Video Broadcasting (DVB), certain fields in the ServiceInformation (SI) data are available, such as "original₋₋ network₋₋ ID","transport₋₋ stream₋₋ ID" and "service₋₋ ID" that uniquely identify aparticular service stream in a multitude of data streams. In such asystem, the subscriber unit could communicate all this information tothe VP in order to uniquely identify the digital stream. PID Filter 510in FIG. 5 would then be a more sophisticated filter since it mustprocess all the additional identifier fields in addition to the MPEG-2PID values.

The SI data stream is of course only one example of how programselection data could be distributed to subscriber units. Other protocolscould be employed using the core and/or access networks. Alternatively,program selection data could be distributed via another medium such asfloppy disk or CD-ROM.

There is however data that VP 206 cannot get from the subscriber unitsand must instead obtain from network manager 210. Alternatively, thisdata may be "hard-wired" into the VP if it can be ensured at start ofdeployment that the data will never or seldom change in the future. Asan example, in a network following the DVB standard, this data includesthe list of PIDs that comprise the SI data and the stream rate of the SIdata. This information is required to enable VP 206 to provide the SIstream to any subscriber unit whenever the latter requests it.

The SI stream may be treated as simply another program and sent over theaccess line whenever any subscriber unit requests it. Alternatively, theSI stream may be transparently routed over all the access lines. Thisway, any change in the SI data is automatically available to thesubscriber units. This also simplifies the architecture of VP 206 sinceit need not monitor and determine when the SI stream has been updatedand therefore needs to be routed to the subscriber units. Thedisadvantage of this scheme is that some bandwidth over the access lineswill always be consumed by the SI data. This is also however a drawbackto the prior art video distribution schemes. The SI data may bestructured so as to minimize bandwidth usage.

Network adaptor 502 recovers MPEG-2 packets from the core networkdelivery medium. For example, if core network 208 is based on ATM,network adaptor 502 would perform the appropriate ATM Segmentation andRe-assembly (SAR) functions. Alternatively, if the core network were asatellite channel, network adaptor 502 would include the appropriatechannel decoder, e.g. a QPSK demodulator and Forward Error Correction(FEC) decoder.

As discussed above, VP 206 requires rate and MPEG-2 PID values for thefiltered program from the subscriber units. This is accomplished bysending the information in the control plane via the return channel onthe access line as will be understood by those of skill in the art.However, the same return channel may carry signaling data for a serviceprovider further upstream than VP 206. For example, in a VOD applicationa subscriber unit may send a so-called VCR "trick mode" (pause/fastforward/etc.) command to the server. VP 206 recognizes which data in theupstream channel is addressed to it, and which is to be sent upstream.This function is accomplished within the signaling protocol used betweenVP 206 and the subscriber units.

FIG. 6 depicts return channel data flow within video pedestal 206 inaccordance with one embodiment of the present invention. Each returnchannel within video pedestal 206 includes a check address block 602coupled to a PID filter setup unit 604 that controls the contents of PIDLUT 508. Return signal data intended for VP 206 is identified withincheck address block 602 and forwarded to setup unit 604 so that PID LUT508 required for filtering of incoming MPEG-2 packets may be updatedappropriately. Return signal data intended for upstream components isinstead directed to an upstream multiplexer 606 that aggregates all thesignaling packets to be sent upstream. A program selection monitor 608is optionally provided to log subscriber selections for billing and/orsurvey purposes.

At power-up, subscriber unit 202 requests VP 206 to send SI data (if theSI data is always present on the access line, this step is omitted).Subscriber unit 202 then informs VP 206 as to which MPEG-2 PIDs tofilter in order to receive a specific service.

It will be appreciated that numerous variations of and extensions to thebasic VPN architecture are possible within the scope of the presentinvention. VPN implementation details will depend on a number of factorsincluding settlement patterns and the nature of the existinginfrastructure and communication protocols.

One simple topological variation in the basic VPN scheme is obtained byplacing the VP at the telephone company Central Office (CO), instead ofat a hub in each neighborhood. An advantage of this configuration islower cost since core network 208 is now terminated at the CO. Accessnetwork 204 is still twisted pair based on ADSL, however, the averagelength of each access line is much longer, of the order of 2 or 3 miles.One disadvantage is that current ADSL technology severely limitsbandwidth available on telephone cables of these lengths.

This configuration works very well in conjunction with so-called"scalable" ADSL modems as are envisioned. A scalable modem can operateover variable distances and support different bandwidths. For example,over a 2 or 3 mile distance, the supported bandwidth may be limited to 8or 9 Mb/s; however, as the line length is dropped, the bandwidthincreases. Thus, for example, at a distance of 3000 feet or less, themaximum bandwidth would be 25 Mb/s or higher with scalable modems.

The service can be started using the VP in the CO and with limitedbandwidth to the subscriber unit. However, as services proliferate, theVP can be moved closer to subscriber premises thus increasing theavailable bandwidth, but without any modifications to the existingsubscriber units themselves.

Another group of variations on the basic VPN architecture involvesinterfacing multiple core networks to VP 206. In one alternativeembodiment, multiple OC3 (155 Mb/sec) or OC12 trunk lines (approximately622 Mb/sec) are fed to the same VP 206. VP 206 is then configured tofilter any program from any of these incoming feeds. With this type ofconfiguration, VP 206 can potentially have more available bandwidth thanan entire coaxial cable used in the HFC system.

Another similar configuration employs multiple satellite feeds to VP206. This configuration is particularly useful in the context ofapartment buildings with multiple satellite receiver antennas, eachtuned to a different satellite. The sum total of all availabletransponder bandwidths could easily be of the order of 1000 Mb/s, witheach satellite transponder carrying a bandwidth between 30 and 80 Mb/s.

In the multi-satellite feed case, VP 206 includes a network adaptor foreach incoming transponder signal. There may be feeds from as many as,e.g., 50 different transponders each with an average bandwidth of 30Mb/s. If it is desirable to limit the number of feeds coming into VP206, the MPEG-2 streams from individual transponders can be combinedwithin an MPEG-2 re-multiplexer prior to input to VP 206.

FIG. 7 depicts a multiple-feed VP architecture based on a cross-barswitch in accordance with one embodiment of the present invention. Amulti-feed VP 702 is implemented using a simple crossbar switchedconnection. The inputs to multi-feed VP 702 are a set of input feeds 704corresponding to, e.g., input trunk lines and/or satellite receiveroutputs. The outputs of multi-feed VP 702 are a set of access lines 706.Each access line 706 receives its data from a filter/modulator 708 whichperforms the functions discussed in reference to FIG. 5. A series ofswitches 710 connects each input feed 704 to an access line 706. Onlyone of the switches 710 is on for any access line 706 at any time. Thecorresponding MPEG-2 stream is filtered and delivered to the outputfilter/modulator 708.

As previously described, the subscriber unit to VP signaling protocol isexpanded accordingly to implement "feed-selection" in VP 702. Thesubscriber unit additionally informs VP 702 as to which input feed toconnect to. It does this, for example in the context of a DVB compliantnetwork, by providing VP 702 with the complete set of identificationparameters for the required MPEG-2 stream ("original₋₋ network₋₋ ID","transport₋₋ ID", and "service₋₋ ID"). The subscriber unit receives thisinformation as a part of network information within the SI data as isknown to those of skill in the art. VP 702 interprets this informationto determine which input feed 704 to select for the specific subscriberunit. Again, as mentioned previously, the mapping between theinformation provided by the subscriber unit and the input feed may bestatic, i.e. hard-wired in VP 702, or it may be under the control of anetwork manager which can change it dynamically.

Another variation concerns filtering of desired program material whichneed not be limited to the MPEG-2 layer based on the unique identifiervalues of MPEG-2 packets. The concept of extending the tuning upstreaminto the network may be implemented in other ways as well. For example,in one embodiment in which the MPEG-2 packets are transported to thesubscriber unit over ATM cells, the filtering of digital data in thepedestal may be performed by selecting the appropriate ATM cells. SinceATM cells are uniquely identifiable by their Virtual Path Identifier(VPI) and Virtual Connection Identifier (VCI) values, the VP merelyselects the ATM cells with the appropriate VPI/VCI values from theincoming multitude of ATM cells.

In this case the subscriber unit has to signal the correct values of theVPI/VCI instead of MPEG-2 stream identifier values to the VP.Alternatively, the VP may have the intelligence to map the MPEG-2 streamidentifiers values received from the subscriber unit to the correctVPI/VCI values.

In either case, the task is simplified if each MPEG-2 service has aunique VPI/VCI value associated with it. For example, in one embodimentwhich is a DVB compliant system, the Service Information datastandardized by DVB is further extended to also include VPI/VCI valuesfor each service. This information is received by the subscriber unitand conveyed to the VP.

If it is not possible to ensure that each MPEG-2 service has a uniqueVPI/VCI then the structure of the VP becomes more complex since it nowhas to monitor the MPEG-2 PID values buried inside the incoming ATMcells to determine which cells to relay to the subscriber unit. This maybe even more complicated by the fact that MPEG-2 packets belonging todifferent services may be packed together inside the same ATM cell. Theadvantage of such an embodiment, of course, is that the entire MPEG-2layer of the transport protocol is independent of the networkarchitecture. For example, this makes the task of generating ServiceInformation independent of network architecture.

Other embodiments that do not rely on MPEG-2 can also take advantage ofthis invention, as can be appreciated by those of skill in the art.Exemplary embodiments of the present invention include the appropriatenetwork architecture with adequate signaling protocol support, asdescribed above, a mechanism to uniquely identify each digital streamavailable from the source, and a mechanism to convey the streamidentifier information to the entity that is desirous of selecting anyspecific stream from the network or the source. In the specific case ofa DVB compliant MPEG-2 based system, the stream identifier informationis carried in the SI stream and is made available to all entities thatwish to select a specific digital stream from the network. A non-MPEG-2system could use other means to convey the same information that iscontained in the SI stream of a DVB system, for example, through afloppy disk. The basic concept of shifting tuning into the network inaccordance with the present invention may be extended to implement ahierarchical VPN architecture, with multiple VP's regulating the flow ofdata through the network. Any component such as a VP or a subscriberunit may transmit a signal upstream, i.e., toward the broadband digitalinformation source, to request a specific digital service from amongstthe multitude of digital services that are available in the network. Ofcourse, it is obvious from the discussion so far that all suchcomponents should have the ability to uniquely identify the digitalstream they want to receive. In one embodiment such information istransmitted in the downstream direction, i.e., away from the broadbanddigital information source, by the server or other entity like networkmanager. Specifically, the SI stream in a DVB compliant architecturecarries this information. Thus, all components that are desirous ofselecting streams from a location upstream in the network should becapable of decoding and interpreting the SI stream.

In the simplest implementation, a signal requesting a specific digitalservice may be allowed to travel upstream along all available pathsuntil it reaches the servers wherein it is terminated. All VPs traversedby the signal set their stream filters to pass the desired streamdownstream. Of course, if any VP already has its filters appropriatelyset, it need not respond explicitly to the message.

One potential disadvantage of such an implementation is the possibilityof "flooding" in the upstream channel since all signals are blindlybroadcast along all available upstream paths. This problem is eliminatedby building additional intelligence into the VPs as follows. Each VPdetermines if it is already passing the requested stream through to theoutput. If yes, it terminates the signal. If no, the VP sets its streamfilters appropriately and then determines along which upstream pathamongst its multiple input feeds should the signal be propagated andsends the signal along that path. Determination of the suitable upstreampath is done by maintaining a list of identifiers for all streams thatare available in each input feed and comparing this list with the streamidentifier information in the received signal. For example, in a DVBcompliant architecture, the list may contain the "original₋₋ network₋₋ID's" of all the networks that are connected to a particular input feed.The "original₋₋ network₋₋ ID" of the requested stream is then matchedwith the appropriate input feed. In more complex networks additionalinformation like "transport₋₋ ID" and "service₋₋ Id" may also berequired to be matched.

The list of stream identifiers for each input feed may be hard-wiredinto the VP if it known before hand that they will not be altered.Alternatively, an external entity like a network manager may update thelist dynamically.

In accordance with the invention, bandwidth available within ahierarchical VPN may be opportunistically exploited by supplementaryservices, video or otherwise by employing "statistical multiplexing" asherein described. FIG. 8 depicts an exemplary two layer VPN 800 suitablefor implementing statistical multiplexing in accordance with oneembodiment of the present invention. VPN 800 includes a core VP 802, anaccess VP 804, a representative subscriber unit 806. Feeds 808 are theinput to core VP 802. An OC48 line 810 interconnects core VP 802 andaccess VP 804.

Consider the use of VPN 800 to implement an NVOD service. For thepurposes of this example it is assumed that OC48 line 810 can deliverapproximately 480 channels of video programming. (The actual number ofchannels that can be delivered over any band-limited network is afunction of the data rate allocated to each channel as well as the totalcapacity of the network.) This is sufficient for an NVOD serviceconsisting of 20 movies, each of 2 hours duration, and with a staggerinterval of 5 minutes. Any additional service would require morecapacity on the connection between the core VP 802 and access VP 804.

Statistically speaking, it is improbable that every one of the 480available channels will be viewed by some subscriber served by access VP804 at all times. At any one moment, some of the channels would not haveany subscriber tuned to them, and their bandwidth would essentially bewasted.

In accordance with the statistical multiplexing feature of the presentinvention, these channels could instead be used to provide additionalservices. For example, if a particular user did not want to watch any ofthe movies on the NVOD service, he/she could be offered some otherservice (e.g., interactive home shopping or VOD or even a program fromanother broadcast/NVOD service) on one of the NVOD channels notcurrently being viewed by anybody. These "opportunistic" servicesdirectly contribute to the revenue stream of the service provider.

Since the underlying model here is statistical, there will be occasionswhen situations arise so that statistical multiplexing could result indegradation of service quality. For example, consider the case when auser decides to take a break from watching a movie knowing that she/hecan return to the same point in the movie by tuning to another channelwhich has the same movie delayed by the appropriate time. In themeantime, however, there is a possibility that particular channel wasnot transmitted since nobody wanted to watch it, and its slot in thenetwork is occupied by an opportunistic service. In the worst case, allsuch "freed" NVOD slots could have been grabbed by opportunisticservices with the result that this particular user would not be able toreceive her/his movie at the end of her/his break.

Preferably, an appropriate scheduling and policing mechanism amelioratesthis problem. In one embodiment, the VPN ensures that there is always aminimum number of free channels at any time and does not offer all theavailable channels to opportunistic services.

Referring again to FIG. 8, the network(s) to the left of core VP 802carry very high bandwidth data. There may be one or more core networksfeeding core VP. All the NVOD and other broadcast channels are availableat the input to core VP 802. The SI data for the NVOD and broadcastservice is always routed through to the subscriber units. Using thereceived SI data (which contains the Electronic Program Guide--EPG),each user selects a desired channel for viewing. As in the basic VPNarchitecture depicted in FIG. 3, the appropriate information regardingthe selected channel (stream ID, rate and PID list, etc.) is signaledfrom the subscriber unit to access VP 804 to enable the latter toappropriately tune its filters and route the program through to theuser.

In the absence of statistical multiplexing, this is all the signalingrequired between the subscriber unit and the network. Since all thebroadcast and NVOD channels are available at the input of access VP 804,the latter appropriately sets its filters and subscriber unit 806receives the selected program. However, when statistical multiplexing isemployed, the requested program may or may not be available at the inputto access VP 804. If it is available at the input to access VP 804(which would be the case if some other user had already requested thatprogram), then the access VP needs only set the filters appropriatelyand provide the requested program to the user.

However, if the program is not available at the input to access VP 804(which would be the case if no other user had requested that programuntil that point, or if all the sessions with that program had beenterminated (e.g., the movie had ended) and hence that channel had beenoffered to other opportunistic services), then access VP 804, inaddition to setting its filters appropriately, also bounces the requestto core VP 802. Core VP 802 then adjusts its filters to select therequested program and multiplexes it down the connection to access VP804.

In one embodiment, for simplicity of implementation, the request isalways bounced from access VP 804 to core VP 802, regardless of whetheror not the program is already being filtered by the latter fortransmission to access VP 804. This minimizes the complexity of thesoftware in access VP 804, and reduces the latency in acquiring thechannel. Of course, core VP 802 must then check if it needs to respondto the received signal. The general nature of this signaling protocol(i.e., when it is terminated and when it is propagated upstream) hasbeen described above.

Core VP 802 maintains a list of which channels are currently being usedand which are not. The list of channels not being used is the pool ofavailable slots for the opportunistic services. These lists aredynamically maintained based on information received from the subscriberunits.

In an alternative embodiment, the concept of statistical multiplexing isgeneralized to a network with more than two layers of VPs. FIG. 9depicts statistical multiplexing within a four layer VPN 900 inaccordance with one embodiment of the present invention. VPN 900includes a service provider 902, a subscriber unit 904 and four VPs 906,908, 910, and 912 therebetween. Multiple input and output connectionsare shown for each VP.

In the configuration of FIG. 9, a request from subscriber unit 904 for aparticular program bounces back sequentially from right to left until itreaches a VP which is already filtering the requested program through toits output. Each VP to the right of this VP, i.e., downstream from it,adjusts its filters appropriately and once this process is complete, thesubscriber unit begins receiving the requested program.

The VPs in the network can be understood to be "pseudo-servers" for theNVOD services. When a user requests a program, the signal traversesupstream, i.e., toward the broadband digital information source, untilit reaches the VP where the program is available. That VP then is the"pseudo-server" for that request. Note that the pseudo-server for thenext request for the same program from another user could be a differentVP, depending on the location of that user in the network. The networkdesigner must consider the effects of latency as the number of VPsincreases in the chain.

In accordance with the invention, statistical multiplexing may beapplied to introduce local programming at a neighborhood VP. FIG. 10depicts a VPN 1000 wherein a core VP 1002 is adapted to incorporateinput from a local server 1004 in accordance with one embodiment of thepresent invention. VPN 1000 includes core VP 1002, an access VP 1006, arepresentative subscriber unit 1008, an OC48 interconnection 1010between core VP 1002 and access VP 1006, and access lines 1012representing the outputs of access VP 1006. Core VP 1002 includesmultiple feeds 1014 including a feed from a local server 1004. Server1004 could be used, for example, to opportunistically inject programmingand/or commercials relevant to a neighborhood or community. Anysubscriber unit in the neighborhood can access this server.

In accordance with the invention, statistical multiplexing may also beapplied in the context of a shared physical medium, e.g., a loop or busarchitected access network such as an HFC network. This configuration isparticularly useful to the operators of cable TV systems or others whoalready have such a network in place.

Every subscriber unit connected to the loop access network potentiallyhas access to all the data being transmitted. Typically, encryption isused to protect data from unauthorized use.

The prior art scheme for providing NVOD services over HFC is to simplybroadcast all the programming over the cable, consuming as muchbandwidth as is required but limited, of course, by the total availablebandwidth. Each subscriber unit can access all these programs, howeverit can decode only what it is authorized to. By introducing a VP at theheadend (which is the source of all the services in the HFC cable),access to the data transmitted over the HFC is restricted to only thoseservices that have been requested by the subscriber units. Services thathave not been requested are not transmitted on the cable. In accordancewith the invention, capacity left unused due to unrequested services canthen be used for opportunistic services, including accessing programsfrom a local server in the headend.

In accordance with the present invention, standard HFC spectrumallocations may be modified to minimize latency when selectingprogramming. Typical HFC implementations allocate only a small portionof the spectrum (of the order of 25 MHz near the low end) for upstreamsignaling from the subscriber units. Due to the limited bandwidth here,which is shared between all the users, the signaling latency istypically very large, and would cause delays in selecting programming.In accordance with the invention, the upstream signaling bandwidth isincreased to reduce this latency. Since statistical multiplexingincreases the efficiency of the downstream allocation, the spectrumreallocated for upstream signaling does not have an appreciable impact.

In an alternative embodiment of the present invention, the VP in an HFCenvironment implements a logical star topology over the existingphysical loop topology. Assuming that there are at least as manychannels (herein a channel is defined as the minimum network capacitythat is needed for delivering any one of the total available services)available on the cable as there are subscriber units connected to it, aspecific channel is dedicated to each subscriber unit, and thesubscriber unit tunes this channel to the appropriate service availableat the VP in the headend. This architecture is structurally similar tothe telephone network-based VPN discussed in reference to FIGS. 2-8.

As compared to the telephone network, however, the cable implementationhas restricted capacity. In a telephone network VPN used for broadcastand/or NVOD, any number of subscriber units may be connected to a VP andoffered services without any drop in quality as long as the trunkcapacity feeding the VP is sufficient to carry all the programming. Ofcourse, the size of the VP will increase with the number of users.However, for the logical star HFC architecture, once the number ofsubscriber units connected to the cable equals the cable capacity, i.e.,the number of channels available over the cable, the network cannot addany more users, even with the same NVOD service offerings. The logicalstar HFC architecture, is thus most useful in the context of highbandwidth implementations which rely heavily on fiber optics fordistribution.

In accordance with one embodiment of the present invention, statisticalmultiplexing is extended to optimizing use of the output bandwidth of anNVOD server. The output bandwidth that is made available by thisoptimization then becomes available for other services. For the purposesof this discussion, an NVOD server is a device that stores digitalinformation in the form of multiple digital information streams orcontent streams. A content stream is a stream of data such as videoprogramming and/or other data such as computer software, audio data,stock market quotes, etc. Since the content streams are not restrictedto video, an NVOD server may be understood to be a digital informationstream server. A given content stream is transmitted over multiplechannels such that the start time for the content stream on each channelis staggered relative to other channels by a predetermined amount oftime. A subscriber requesting a given content stream must wait until thenext such starting time and tune to the channel having that startingtime for the requested content stream.

FIG. 11 depicts a simplified representation of an NVOD server 1100modified in accordance with the invention. As will be explained below,an NVOD server modified in this fashion will be referred to herein as anon-demand NVOD server or OD-NVOD server. OD-NVOD server 1100incorporates NVOD program material storage 1102, opportunistic servicematerial storage 1104, a scheduler 1106 which implements schedulingfunctions described below, a network adaptation unit 1108 which actuallytransmits the correct program material on the requested channel, and arequest processor 1110 which handles requests from subscriber units ordownstream VPs. An input 1112 is also provided to receive externalsources of opportunistic services. The structure depicted in FIG. 11 isillustrative only and the functional units depicted could be implementedin either hardware or software.

FIG. 12A depicts OD-NVOD server 1100 coupled to the hierarchical VPN ofFIG. 8. Statistical multiplexing may be applied to the output of an NVODserver whether it is used with a VPN or is coupled to subscriber unitsin some other way. FIG. 12B depicts NVOD server 1100 coupled to a seriesof subscriber units 1204 by a cable or HFC distribution system 1206without use of a VPN. A very low bandwidth return channel is provided toreturn tuning information from the subscriber units.

FIG. 13A depicts the operation of a prior art NVOD server. Prior artNVOD servers store a number of content streams and transmit them atstaggered start times. For example, a movie of 2 hours duration may bestreamed out on 24 channels with a five minute staggered intervalbetween the start times on different channels. The prior art NVOD serverincorporates a scheduler which automatically commences transmission ofthe movie at an appropriate time. This way a user interested in themovie has the flexibility to watch the movie at his/her convenience.

In accordance with the invention, NVOD server 1100 may be modified toincorporate statistical multiplexing capabilities to optimize usage ofits output bandwidth. If 24 channels are broadcasting the same movie atstaggered intervals, it is likely that at least some of these channelsare wasted since no user is tuned toward them. In accordance with theinvention, instead of simply streaming out the same content data onmultiple channels, the intelligent NVOD server now streams out thecontent on only as many channels as have been demanded. The remainingchannels allocated to that content are then available for offeringopportunistic services. The information for these services may either bestored internally to the server or be available externally. An NVODserver modified in this fashion is thus referred to herein as an"on-demand" NVOD server, or OD-NVOD server.

Software-based scheduler 1106 maintains relevant schedule informationfor each output channel. However, the OD-NVOD server does not controlits output in response to the schedule alone. Instead, the OD-NVODserver is responsive to requests for programming as received by requestprocessor 1110. A video program is not transmitted at the scheduled timeunless there is a specific request for it. This request could originatefrom a subscriber (i.e., the channel carrying the video program isselected) or from a network operator that processes subscriber requests.In the VPN context, the request may be received via one or more VPs.

Often, a content stream will be requested after its scheduled starttime. The OD-NVOD preferably handles such a request by initiatingtransmission of the program not at the beginning of the program, but atthe point at which the program would have been had the program startedat its scheduled start time. Of course, the exact point in the programat which the request is received may be difficult to determine preciselyand thus may be approximated. Additionally, it may be advantageous tobegin streaming the data from a convenient MPEG synchronization pointclose to the requested point to enable rapid decoder synchronization.

FIG. 13B depicts the operation of the NVOD server in response to userrequests for a particular program. Consider the activity on channel 3.The scheduled start time for this channel is t=10 minutes. However, ifno subscriber has requested this channel, the OD-NVOD will not beginstreaming data at t=10 minutes. Assume now that a subscriber tunes tothis channel at t=24 minutes. Since the program is not currently beingtransmitted by the NVOD server, the request for the program bounces backfrom the subscriber to the server through the VPN. The OD-NVOD serverupon receiving the request begins transmitting the program on channel 3.However, transmission of the program commences not at the beginning butrather at a point 14 minutes into the program (this is the point wherethe program would have been at the time the program is placed on theoutput (i.e., 24 minutes) had it indeed started playing at t=10 minutes.

Note that for the purposes of this discussion, the term "channel" simplyrefers to an allocated portion of the NVOD server output capacity ornetwork transmission capacity necessary to transmit a single videoprogram in real time. Thus, a channel could represent an MPEG packetstream, an ATM virtual connection, or a spectral allocation. Forexample, if the output interface of the server is ATM-based, eachchannel is mapped to a specific VPI/VCI at the ATM layer. The VPI/VCIfields in the ATM cells are used to differentiate this channel from anyother channel when transmitted over a common network. The actual VPI/VCImapped to a particular channel may change dynamically, and it is theresponsibility of some other entity, to ensure that the settop units areupdated properly to allow tuning. In fact, it may well be the case thatthe same channel is mapped to different VPI/VCI values before the movieends. For example, referring again to FIG. 13B, channel 4 may initiallybe mapped to a specific VPI/VCI value, however, when its transmission isresumed at approximately t=112 minutes after being turned-off at t=87minutes, it may be assigned a different VPI/VCI. This may happen becausethe original VPI/VCI assigned was assigned to another service during theinterval when this channel was turned off.

Those of skill in the art will appreciate that some latency will beintroduced between when the request is received and when transmission ofthe program is initiated. To simplify the discussion, this latency isignored here.

Further efficiency in bandwidth usage is obtained by ceasingtransmission of programs once they are no longer being viewed by anysubscriber. Referring again to FIG. 13B, at t=85 minutes, the OD-NVODserver learns that the last user has tuned off channel 4 and responds byceasing transmission of channel 4, again making this channel availablefor opportunistic services.

For a network as in FIG. 12B where the OD-NVOD is in directcommunication with the settop units, request processor 1110 maintainsfor each channel a list of settop units that have requested the programtransmitted over that channel. When a settop unit ceases to monitor aprogram (or is powered off), it sends a cancellation request to theOD-NVOD server. The OD-NVOD server responds to the cancellation requestby deleting that settop unit from the list of requesters. Once the listof requesters for a particular channel is empty, the OD-NVOD serverceases transmission on that channel. The OD-NVOD server may check if thelist is empty either periodically or whenever a cancellation request isreceived.

In a hierarchical VPN, the process of turning off a channel is somewhatmore complex. Each VP determines if a specific channel available at itsinput is also being passed through downstream, i.e., if it is beingfiltered by that VP. If a particular channel is not being filtered itimplies that no subscriber downstream from that VP is tuned to thatchannel, and consequently that channel is not required at the input ofthis VP. The VP then signals to the next VP in the upstream directionfrom which it is receiving that channel to halt transmission of thatstream. (The process of determining which VP is the source of aparticular stream is performed by matching various stream identifierfields, e.g., "original₋₋ network₋₋ ID" in the case of a DVB compliantsystem, as described above.) The upstream VP responds to the signal byhalting further transmission of the channel, and consequently, theportion of the bandwidth between the two VPs that was previouslyoccupied by the channel is now available to other opportunisticservices.

Each VP in the network, beginning with the VPs which are directlyconnected to the settop units, repeats the above described process ofdetermining whether any channel is not currently in use, and if sosignaling a cancellation request to the next upstream VP to halttransmission of that channel. This checking process may be prompted bythe receipt of a cancellation request from a downstream VP or STU or mayoccur periodically. The process stops at the VP which finds at least oneuser downstream from it that is monitoring the channel, i.e., is tunedto the channel. Ultimately, if no subscriber is tuned to a particularchannel, the process bounces through all the VPs to the OD-NVOD serverwhich turns off that channel.

To implement this turn-off function, the OD-NVOD server maintains a listof VPs to which it is coupled which have requested a particular channel.When a VP sends the OD-NVOD server a cancellation request, the OD-NVODserver deletes the VP from the requester list. Once the requester listfor a particular channel is empty, the OD-NVOD server ceasestransmission on that channel. The OD-NVOD server may check the list tosee if it is empty upon receipt of a cancellation request orperiodically.

Preferably, the VP or OD-NVOD server waits a pre-determined length oftime after the last user has tuned off from a channel before concludingthat no subscriber is utilizing that channel. This wait is useful toavoid false alarms due to accidental channel changes or so-called"channel surfing."

OD-NVOD output bandwidth that is left free because no subscriberrequires it, is available for opportunistic services. Many such servicesare possible including promotional material, digital informationrequested by a particular subscriber, and other programming. Thisprogramming material may be stored within the OD-NVOD server or beavailable to it.

In one embodiment, there are two classes of subscribers, privileged andnon-privileged. Accordingly, OD-NVOD server 1100 maintains inconjunction with scheduler 1106 a database that indicates whichsubscribers are privileged and which subscribers are non-privileged. Inan alternative embodiment, the database may be maintained by an entityexternal to the OD-NVOD server, e.g., a Subscriber Management System--orSMS--as is known to those of skill in the art. Scheduler 1106 honorsrequests for NVOD material only when they originate with privilegedsubscribers. Opportunistic programming material is available to allsubscribers but is of course only available on a particular channel whenNVOD material is not currently being transmitted to fulfill a request bya privileged subscriber. Alternatively, in the hierarchical VPN case,one or more VPs in the chain between the subscriber and the OD-NVODserver perform the function of rejecting NVOD program material requestsfrom non-privileged subscribers.

The advantages of the above-described digital video distribution schemeincorporating statistical multiplexing of OD-NVOD output along withdifferentiation of two classes of subscribers will be made apparent withreference to two specific examples of revenue generating services. Inboth examples, the privileged subscribers have paid for access to NVODprogram materials, e.g., movies.

In the first example, if no privileged subscriber is tuned to aparticular NVOD channel, the OD-NVOD server will not stream out themovie designated for this channel. Instead, the OD-NVOD server willtransmit other data that is freely available to all customers asavailable from opportunistic program storage 1104 or input 1112. Forexample, a stream containing promotional material could be broadcast onthis channel. In this case, when a non-privileged subscriber tunes tothe channel, a request for the promotional material will beautomatically generated and he or she will see the promotional materialinstead of a blank screen as would be provided in prior art systems. Theeconomic benefit to the service provider is clear since channels thatwould typically not be available to a non-privileged customer can now beused to generate revenues by providing him/her with promotionalmaterial. Of course, as soon as any privileged subscriber tunes to thischannel, the OD-NVOD server receives a request and switches from thepromotional content to the movie stream. The non-privileged subscriberthen loses her/his picture and may instead see a blank screen. Thus, theOD-NVOD server can be used more profitably than prior art NVOD servers.

In the second example, the non-privileged subscriber may take advantageof unused NVOD channels to download content, e.g., computer data, fromthe OD-NVOD server as available from opportunistic program storage 1104or input 1112. Of course, there is the possibility that if a privilegedsubscriber tunes to the channel before all the data has been downloaded,the latter may lose her/his data (this is because the NVOD material hashigher priority). The advantage of this scheme over the scheme of thefirst example is that a non-privileged subscriber need not watch thepromotional material broadcast by the service provider, but can insteadreceive services of her/his choice for the duration the channel isavailable. Of course, in such a situation, it is possible that onenon-privileged subscriber may monopolize usage of the opportunisticchannel. In an alternative embodiment, there are different levels ofpriority among non-privileged subscribers requesting opportunisticservices, and a request from a higher priority subscriber displaces onefrom a lower priority subscriber.

The techniques described above for optimizing usage of NVOD serveroutput bandwidth may be further generalized to other broadcast equipmentsuch as an MPEG multiplexer as would typically be installed at a cableheadend. FIG. 14 illustrates an MPEG multiplexer 1400 configured inaccordance with the invention. Multiplexer 1400 is connected to ascheduler 1402 and a request processor 1406 as well as a networkadaptation unit 1408. Multiplexer 1400 receives as input several moviesand at least one commercial stream as well as a stream containing otherservices and combines them into a multiplexed stream to transmit tonetwork adaptation unit 1408 for relaying to subscribers.

In the prior art, a scheduler controlling the multiplexer wouldperiodically remove one of the movie feeds from the multiplexed streamand substitute the commercial stream. Alternatively, this could occurunder external control. At the appropriate time, the movie isresubstituted for the commercial stream.

In accordance with the invention, statistical multiplexing is applied toenable "on-demand" transmission of content. As before, scheduler 1402may maintain an internal database indicating when the commercial streamis to be substituted for a particular movie feed. However, scheduler1402 does not insert a movie feed in the multiplexed stream unlessrequest processor 1406 receives at least one request from a subscriberunit for that particular feed. Instead a commercial stream is insertedinstead. Once a request is received, the movie stream is switched inusing the MPEG PID previously used by the commercial stream.Alternatively, some other service requested by a user could be insertedinstead. In one embodiment, only privileged subscriber units areentitled to request and receive the movie feed but all subscriber unitsmay receive the commercial stream.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. It should be evident that the present invention is equallyapplicable by making appropriate modifications to the embodimentsdescribed above. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the metes andbounds of the appended claims.

What is claimed is:
 1. In a digital communication system including aplurality of subscriber units, a digital information transmission systemcoupled to said plurality of subscriber units by a communicationnetwork, comprising:means for transmitting a digital information streamof a predetermined duration to one or more of said subscriber units viasaid communication network over a predetermined channel beginning at apredetermined time; and scheduling means coupled to said transmittingmeans forif a request for said digital information stream is receivedprior to said predetermined time, initiating transmission of saiddigital information stream at said predetermined time starting at abeginning of said digital information stream over said predeterminedchannel; and if said request for said digital information stream isreceived after said predetermined time, initiating transmission of saiddigital information stream at a point in said digital information streamdetermined relative to said predetermined time so that said digitalinformation stream ends said predetermined duration after saidpredetermined time.
 2. The system of claim 1 wherein said request isreceived from a subscriber unit.
 3. The system of claim 2 wherein saidplurality of subscriber units includes privileged and non-privilegedsubscriber units and said scheduling means accepts requests for saiddigital information stream only from privileged subscriber units.
 4. Thesystem of claim 1 wherein said scheduling means further comprises meansfor:if no request for said digital information stream is received,accepting requests for an alternative digital information stream on saidchannel, and using said transmitting means to transmit informationrelating to said alternative digital information stream over saidchannel.
 5. The system of claim 4 wherein said scheduling means furthercomprises means for:upon receipt of a request for said digitalinformation stream within said predetermined duration after saidpredetermined time, terminating transmission of information relating tosaid alternative digital information stream.
 6. The system of claim 1wherein said scheduling means further comprises means for:if no requestfor said digital information stream is received, initiating transmissionof an alternative digital information stream over said channel from saidpredetermined time until said request for said digital informationstream is received.
 7. The system of claim 6 wherein said schedulingmeans further comprises means for:upon receipt of a request for saiddigital information stream, terminating transmission of said alternativedigital information stream.
 8. The system of claim 1 wherein saidcommunication network is a hierarchical network interconnecting saiddigital information transmission system and said plurality of subscriberunits.
 9. The system of claim 1 wherein said communication network is ashared communication medium interconnecting said digital informationtransmission system and said plurality of subscriber units.
 10. Thesystem of claim 1 wherein said transmitting means is an MPEGmultiplexer.
 11. The system of claim 2 wherein said scheduling meansfurther comprises:means for, once transmission of said digitalinformation stream has been initiated, receiving messages from furthersubscriber units indicating that said digital information stream isbeing monitored and receiving further messages from subscriber unitsindicating that said digital information stream is no longer beingmonitored, and upon a determination that no subscriber unit of saidplurality is currently monitoring said digital information stream,thereafter terminating transmission of said digital information streamand accepting requests for transmitting alternative services over saidchannel.
 12. The system of claim 1 wherein said channel is an MPEGmultiplexed packet stream allocation.
 13. The system of claim 1 whereinsaid channel is a frequency allocation.
 14. The system of claim 1wherein said channel is an ATM virtual connection.
 15. A digitalinformation distribution system comprising:a digital information streamserver comprising:means for storing a digital information stream ofpredetermined duration; network adaptation means for transmittingdigital information onto a first communication network on apredetermined channel; request receiving means for receiving requestsfor said digital information stream from said first communicationnetwork; scheduling means for directing said digital information streamto said network adaptation means for transmission over said firstcommunication network on said predetermined channel at a predeterminedtime, if a request for said digital information stream is received bysaid request receiving means; and opportunistic programming means fordirecting digital information to said network adaptation means fortransmission over said first communication network only if said digitalinformation stream is not being transmitted, wherein said schedulingmeans further comprises means for:if a request for said digitalinformation stream is received prior to a predetermined time, initiatingtransmission of said digital information stream starting at a beginningof said digital information stream over said predetermined channel atsaid predetermined time; and if said request for said digitalinformation stream is received after said predetermined time, initiatingtransmission of said digital information stream at a point in saiddigital information stream determined relative to said predeterminedtime so that said digital information stream ends said predeterminedduration after said predetermined time.
 16. A digital informationdistribution system comprising:a digital information stream servercomprising:means for storing a digital information stream ofpredetermined duration; network adaptation means for transmittingdigital information onto a first communication network on apredetermined channel; request receiving means for receiving requestsfor said digital information stream from said first communicationnetwork; scheduling means for directing said digital information streamto said network adaptation means for transmission over said firstcommunication network on said predetermined channel at a predeterminedtime, if a request for said digital information stream is received bysaid request receiving means; and opportunistic programming means fordirecting digital information to said network adaptation means fortransmission over said first communication network only if said digitalinformation stream is not being transmitted, wherein said opportunisticprogramming means transmits digital information via said channel onlyupon request of a subscriber unit when said digital information streamis not being transmitted.